Method and system for performing channel estimation in a multiple antenna block transmission system

ABSTRACT

Methods and systems for performing channel estimation in a multiple antenna block transmission system are provided so as to improve the channel estimation quality and/or the delay spread of the channels that can be estimated.

RELATED APPLICATIONS

This application is a divisional application of patent Ser. No.11/195,523 filed Aug. 1, 2005, now issued as U.S. Pat. No. 7,382,842.

BACKGROUND

The invention relates to block transmission systems. More specifically,the invention relates to a method and system for performing channelestimation in a multiple antenna block transmission system.

In some known block transmission systems, for example, it is desirableto transmit a preamble from a first transmit antenna for synchronizingand training receivers or subscriber units that are capable of decodingsignals from a single transmit antenna. However there may be othersubscriber units that can decode signals from multiple transmitantennas. In order to train these remaining transmit antennas to thesame extent as the first transmit antenna, considerable overhead isincurred. These remaining transmit antennas may transmit with lowerpilot density to avoid these overheads. This, in turn may limit thedelay spread of the channels that can be estimated or effect the channelestimation quality for these remaining transmit antennas.

In some block transmission systems, the quality of the pilots of a firsttransmit antenna may be different from the quality of the pilots ofremaining transmit antennas, due to different boosting, differentinterference (as seen at the receiver) or other factors. This in turneffects the quality of the channel estimation corresponding to the firsttransmit antenna.

Therefore there is need for a method and system that improves thechannel estimation quality and increases the delay spread of thechannels that can be estimated in such block transmission systems.

SUMMARY

An objective of the invention is to provide a method and system forperforming channel estimation in a multiple antenna block transmissionsystem, in which the delay spread of the channels that can be estimatedis increased to that of at least one first transmit antenna (havingsignificantly higher pilot density among a plurality of transmitantennas).

Another objective of the invention is to provide a method and system forchannel estimation in a block transmission system, in which the channelresponses are estimated based on at least one weighted averaged delayprofile so as to improve the channel estimation quality.

In order to fulfill the above-mentioned objectives, a method and systemfor performing channel estimation for each transmit antenna-receiveantenna pair in a block transmission system is provided.

In an embodiment of the invention, the method includes estimating atleast one first delay profile for each receive antenna corresponding toat least one first transmit antenna of a transmitter. The pilot densityof the at least one first transmit antenna is significantly higher amonga plurality of transmit antennas of the transmitter. The method furtherincludes estimating at least one desired delay profile for each receiveantenna corresponding to each remaining transmit antenna of thetransmitter based on the at least first delay profile. Each remainingtransmit antenna has pilot density lesser than the at least one firsttransmit antenna. The method further includes determining the channelresponse of each receive antenna corresponding to each remainingtransmit antenna based on the corresponding at least one desired delayprofile. As a result, the delay spread of the channels corresponding tothe remaining transmit antennas that can be estimated is extended tothat of the at least one first transmit antenna.

In another embodiment of the invention, the method includes determininga delay profile of each transmit antenna-receive antenna pair andcalculating at least one weighted averaged delay profile. The method ofobtaining the weighted averaged delay profile comprises performing aspatial weighted averaging of the delay profiles of the plurality oftransmit antenna-receive antenna pairs. The method further includesestimating a channel response of each transmit antenna-receive antennapair based on the at least one weighted averaged power delay profile.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and the attendedadvantages will become readily apparent as the same becomes betterunderstood by reference of the following detailed description whenconsidered in conjunction with the accompanying drawings in whichreference symbols indicate the same or similar components, wherein:

FIG. 1 is a flowchart of a method for performing channel estimation foreach transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with an embodiment of the invention.

FIG. 2 is a flowchart of a method for performing channel estimation foreach transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with another embodiment of the invention.

FIG. 3 is a block diagram of a system for performing channel estimationfor each transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with an embodiment of the invention.

FIG. 4 is a block diagram of a system for performing channel estimationfor each transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF DRAWINGS

This invention provides a method and system for performing channelestimation for each transmit antenna-receive antenna pair in a blocktransmission system. Examples of block transmission include OrthogonalFrequency-Division Multiplexing (OFDM), Multi-Carrier Code DivisionMultiple Access (MC-CDMA), Discrete Multi-Tone (DMT) and the like. TheIEEE 802.16d and 802.16e wireless Metropolitan Area Network (MAN)standards, which use Orthogonal Frequency Division Multiple Access(OFDMA) (an OFDM technology with multiple access) also fall in thiscategory.

FIG. 1 is a flowchart of a method for performing channel estimation foreach transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with an embodiment of the invention. The blocktransmission system includes at least one transmitter. In thisembodiment of the invention, the transmitter includes a plurality oftransmit antennas. At least one first transmit antenna has significantlyhigher pilot density, or the pilot spacing available for the at leastone first transmit antenna is smaller than that available for theremaining transmit antennas. This occurs for example, in 802.16d and802.16e WMAN OFDMA systems, where each frame comprises a preamble thatis transmitted only by a first transmit antenna. The preamble has pilotsthat are spaced three sub-carriers apart. The pilot density in a symbolfor the remaining transmit antennas is much lower. In general, greaterthe pilot density (or the number of pilots divided by the total numberof sub-carriers used), better is the quality of channel estimate.

At step 105, at least one first delay profile of each receive antenna isestimated corresponding to at least one first transmit antenna. A delayprofile of a transmit antenna-receive antenna captures information aboutthe delays at which dominant paths or taps in the channel impulseresponse (CIR) exist. Due to the nature of wireless channels, the CIR isspecular in nature. In other words, significant energy in the taps existonly at certain delays and not all delays.

In an embodiment of the invention, the first delay profile is a firstpower delay profile (PDP). A PDP (ξ_(m,n)) of a transmit antenna(m)-receive antenna (n) pair provides information on the tap powerversus the tap delay of the CIR of the transmit antenna-receive antennapair. In an embodiment of the invention, the PDP is obtained as|h_(m,n)(k,l)|^2,

-   where,-   h is channel impulse response;-   k is a tap index; and-   l is a symbol index.    In another embodiment of the invention, it is obtained by also    performing a weighted time averaging of |h_(m,n)(k,l)|^2    corresponding to a plurality of symbols (l). In one embodiment,    these weights are chosen to be equal. In another embodiment, the    weights are chosen so that a symbol l is given higher weightage than    a symbol l-p, where p is a positive integer.

In another embodiment of the invention, the first delay profile is afirst tap delay profile (TDP). A TDP (λ_(m,n)) of a transmit antenna(m)-receive antenna (n) pair provides the information regarding theposition or location of the significant taps in the corresponding CIR.In an embodiment of the invention, the TDP is estimated by obtaining thePDP and then considering those tap locations where the power in the PDPexceeds a threshold. The threshold is a design parameter. In anotherembodiment, the TDP is obtained from the pilots transmitted by the firsttransmit antenna by using information theoretic criterion like akaikecriterion. One such method is described in a publication by J J Van deBeek et al. entitled, “On channel estimation in OFDM systems” in Proc.of IEEE VTC, 1995, vol. 2, pp. 815-819, July 1995.

In yet another embodiment of the invention, the first delay profile is afirst CIR auto-correlation matrix. An element, (i,j) of theauto-correlation matrix of a CIR is given by h(j)h(j)* (where thesuperscript * denotes complex conjugate). The auto-correlation matrixmay be time weighted averaged across a plurality of symbols in time.

At step 110, at least one desired delay profile is estimated for eachreceive antenna corresponding to each remaining transmit antenna of thetransmitter based on the at least one first delay profile. Eachremaining transmit antenna has a pilot density lesser than the at leastone first transmit antenna.

In an embodiment of the invention, the desired delay profile is adesired PDP (ξ^(d)). In one embodiment, the desired PDP of a receiveantenna (n1), corresponding to each remaining transmit antenna (m) maybe chosen to be a weighted combination of the first PDPs of theplurality of receive antennas, (ξ_(m1,n)) (where m1 denotes at least onefirst transmit antenna having significantly higher pilot density). Inanother embodiment, the desired PDP (ξ^(d) _(m,n1)) of a receive antenna(n1) corresponding to each remaining transmit antenna (m) may be chosento be a weighted combination of the first PDPs (ξ_(m1,n1)) correspondingto the receive antenna (n1). This may be useful, in some cases when theplurality of receive antennas form a diversity array and the channel isslow fading.

In an embodiment of the invention, the desired delay profile is adesired TDP. In an embodiment of the invention, the desired TDP, (λ^(d)_(m,n1)) of a receive antenna (n1) corresponding to each remainingtransmit antenna (m) is chosen as the first TDP of the receive antennacorresponding to a first transmit antenna, (λ_(1,n1)), if the firsttransmit antenna has significantly higher pilot density compared to theremaining transmit antennas.

In another embodiment of the invention, if a first transmit antenna hassignificantly pilot density compared to the remaining transmit antennas,the desired TDP of a receive antenna (n1) corresponding to eachremaining transmit antenna (m) is based on the first TDPs, (λ_(1,n)) ofthe plurality of receive antennas. For example, it may be chosen as theunion across the receive antennas of these first TDPs, (λ_(1,n)). Thismay be useful if the receive antennas form a diversity array and thechannel is fast fading. This may further be useful if the TDP has notbeen derived from the multiple CIRs in time, for example, due to storageor other limitations on the receiver.

In some embodiments, the set of p taps on either sides of each tap inthe desired TDP is also included, prior to obtaining the channelestimates. This is henceforth referred to as smearing. This may help forexample in some cases when the dominant paths are not well separated. Inthese cases, due to the presence of filters in the block transmissionsystem, the resultant CIR may not show significant energy at some of theneighboring tap locations for a particular realization of the gains ofthese dominant taps. However with a different fading realization, thesetap locations also contain significant energy. If the desired TDP isderived from one or few realizations, it may then be smeared to coverthe cases for other realizations.

In yet another embodiment of the invention, the desired delay profile isa desired CIR auto-correlation matrix.

At 115, channel response of each receive antenna corresponding to eachremaining transmit antennas is estimated based on the correspondingdesired delay profile. For example, if the desired delay profile is adesired TDP, the modified Least Square (mLS) method can be used toestimate the channel response for a transmit antenna-receive antennapair, based on the pilots available for the corresponding transmitantenna. One such mLS method is described in (and the referencestherein) by M R Raghavendra and K Giridhar entitled, “Improving channelestimation in OFDM systems for sparse multipath channels”, IEEE SignalProc. Letters, vol. 12, no. 1, pp. 52-55, January 2005.

In various embodiments of the invention, as a result, the delay spreadof channels corresponding to the remaining transmit antennas that can beestimated at the receiver is extended to that of the first transmitantenna. This method helps in performing channel estimation at thereceiver for a remaining transmit antenna, which otherwise may not beestimated due to limited pilot spacing of the remaining transmitantenna. For example, if the pilot spacing available for the firsttransmit antenna is three and the remaining transmit antennas is nine,then the delay spread of the channels corresponding to the firsttransmit antenna that can be estimated is extended from N/9 to N/3 OFDMsamples, where N is the total number of sub-carriers. For example, N isthe Fast Fourier Transform (FFT) size in an OFDM system.

FIG. 2 is a flowchart of a method for performing channel estimation foreach transmit antenna-receive antenna pair in a block transmissionsystem, in accordance with another embodiment of the invention. At step205, a delay profile is determined for each transmit antenna-receiveantenna pair. In an embodiment of the invention, the delay profile is aPDP. In another embodiment of the invention, the delay profile is a CIRauto-correlation matrix.

At 210, at least one weighted averaged delay profile is calculated. Theat least one weighted averaged delay profile is calculated by performinga spatial weighted averaging of the delay profiles of a plurality oftransmit antenna-receive antenna pairs. In an embodiment of theinvention, spatial weighting corresponding to a transmit antenna-receiveantenna pair is determined based on the pilot density of thecorresponding transmit antenna. The spatial weighting corresponding to atransmit antenna-receive antenna pair may also be determined based onthe pilot boosting of the corresponding transmit antenna. The spatialweighting corresponding to a transmit antenna-receive antenna may alsobe determined based on the interferences (as seen at the receiver) onthe pilot locations of the signal transmitted by the correspondingtransmit antenna.

In an embodiment of the invention, a weighted averaged delay profile iscalculated corresponding to each transmit antenna-receive antenna pair.Further, the spatial weights corresponding to a transmit antenna-receiveantenna pair may also be a function of the tap locations. This may beuseful in slow fading channels and when the antennas form a diversityarray. For example, in some cases, if the PDP corresponding to a firsttransmit antenna-receive antenna pair shows no power at a given taplocation, while the PDP corresponding to a second transmitantenna-receive antenna pair shows significant power at the same taplocation, the weightage corresponding to the second transmitantenna-receive antenna pair can be set to zero for this tap location,while obtaining the weighted averaged delay profile for the firsttransmit antenna-receive antenna pair. Also, for determining theweighted averaged delay profile for the second transmit antenna-receiveantenna pair, the weightage corresponding to the first pair, for thistap location, can be set to zero.

In another embodiment of the invention, one weighted averaged delayprofile is calculated corresponding to the plurality of transmitantenna-receive antenna pairs.

In an embodiment of the invention, when the delay profile is a PDP, thePDP computations and weighted averaging is performed on those taplocations where the power exceeds a predetermined power threshold. Thepredetermined power threshold is a design parameter. This helps reducecomputational power.

At step 215, a channel response of each transmit antenna-receive antennapair is estimated based on at least one weighted averaged delay profile.In an embodiment of the invention, a CIR of each transmitantenna-receive antenna pair is estimated based on at least one weightedaveraged delay profile and thereafter, a channel frequency response(CFR) of each transmit antenna-receive antenna pair is estimated basedon the corresponding CIR. For example, the CIR of a transmitantenna-receive antenna pair is estimated from the pilots transmittedfrom the corresponding transmit antenna using the weighted averageddelay profile computed for that pair. In another example, the CIR of atransmit antenna-receive antenna pair is estimated from the pilotstransmitted from the corresponding transmit antenna using the weightedaveraged delay profile that is calculated for the plurality of transmitantenna-receive antenna pairs.

In an embodiment of the invention, the TDPs corresponding to theplurality of transmit antenna-receive antenna pairs is spatiallycombined to obtain a desired TDP for a transmit antenna-receive antennapair. For example, the desired TDP may be the union of the TDPs of theplurality of the transmit antenna-receive antenna pairs. The CFRcorresponding to the transmit antenna-receive antenna pair may be thenobtained from the pilots of the corresponding transmit antenna, usingthe desired TDP. In various embodiments of the invention, the desiredTDP may further be smeared prior to CFR estimation.

FIG. 3 is a block diagram of a system 300 for performing channelestimation for each transmit antenna-receive antenna pair in a blocktransmission system, in accordance with an embodiment of the invention.System 300 includes a first delay profile estimator 305, a desired delayprofile estimator 310, and a channel response estimator 315.

First delay profile estimator 305 estimates at least one first delayprofile of each receive antenna corresponding to at least one firsttransmit antenna of a transmitter. The at least one first transmitantenna has significantly higher pilot density or the pilot spacingavailable for the at least one first transmit antenna is smaller thanthat available for the remaining transmit antennas. In an embodiment ofthe invention, the first delay profile is a first TDP. In anotherembodiment of the invention, the first delay profile is a first PDP. Inyet another embodiment of the invention, the first delay profile is afirst CIR auto-correlation matrix.

Desired delay profile estimator 310 estimates at least one desired delayprofile for each receive antenna corresponding to each remainingtransmit antenna of the transmitter, based on the at least first delayprofile. Each remaining transmit antenna has a pilot density lesser thanthe first transmit antenna. In an embodiment of the invention, thedesired delay profile is a desired TDP. In another embodiment of theinvention, the desired delay profile is a desired PDP. In yet anotherembodiment of the invention, the desired delay profile is a desired CIRauto-correlation matrix.

Channel response estimator 315 estimates the channel response of eachreceive antenna corresponding to each remaining transmit antenna basedon the corresponding at least one desired delay profile. As a result,the delay spread of channels corresponding to the remaining transmitantennas that can be estimated is extended to that of the first transmitantenna.

In various embodiments of the invention, first delay profile estimator305, desired delay profile estimator 310 and channel response estimator315 reside on a receiver. In an embodiment of the invention, thesemodules may interact with one another. In an embodiment of theinvention, first delay profile estimator 305, desired delay profileestimator 310 and channel response estimator 315 can be integrated intoa single module.

FIG. 4 is a block diagram of a system 400 for performing channelestimation for each transmit antenna-receive antenna pair in a blocktransmission system, in accordance with another embodiment of theinvention. System 400 includes a delay-profile-determining module 405, aweighted averaging module 410 and a channel response estimator 415.

Delay-profile-determining module 405 determines a delay profile for eachtransmit antenna-receive antenna pair. In an embodiment of theinvention, the delay profile is a PDP. In another embodiment of theinvention, the delay profile is a CIR auto-correlation matrix. Weightedaveraging module 410 calculates at least one weighted averaged delayprofile. In an embodiment of the invention, weighted averaging module410 includes a spatial averaging module 425. In another embodiment ofthe invention, weighted averaging module 410 includes spatial averagingmodule 425 and a time averaging module 430.

Spatial averaging module 425 performs a spatial weighted averaging ofthe delay profiles of the plurality of transmit antenna-receive antennapairs. In an embodiment of the invention, spatial weightingcorresponding to a transmit antenna-receive antenna pair is determinedbased on the pilot density of the corresponding transmit antenna. Thespatial weighting corresponding to a transmit antenna-receive antennapair may also be determined based on the pilot boosting of thecorresponding transmit antenna. The spatial weighting corresponding to atransmit antenna-receive antenna may also be determined based on theinterferences (as seen at the receiver) on the pilot locations of thesignal transmitted by the corresponding transmit antenna. In anembodiment of the invention, spatial weights corresponding to a transmitantenna-receive antenna pair is also a function of the tap locations.

Time averaging module 430 performs weighted averaging across a pluralityof symbols in time.

In an embodiment of the invention, weighted averaging module 410calculates a weighted averaged delay profile corresponding to eachtransmit antenna-receive antenna pair. In another embodiment of theinvention, weighted averaging module 410 calculates a weighted averageddelay profile corresponding to a plurality of transmit antenna-receiveantenna pairs.

Channel response estimator 415 estimates a channel response of eachtransmit antenna-receive antenna pair based on at least one weightedaveraged delay profile.

In various embodiments of the invention, delay-profile-determiningmodule 405, weighted averaging module 410 and channel response estimator415 reside on a receiver. In an embodiment of the invention, thesemodules may interact with one another. In an embodiment of theinvention, delay-profile-determining module 405, weighted averagingmodule 410 and channel response estimator 415 can be integrated into asingle module.

The various embodiments of the invention provide a method and systemwhereby, if a first transmit antenna has significantly higher pilotdensity compared to the remaining transmit antennas, the delay spread ofthe channels corresponding to the remaining transmit antennas that canbe estimated, is extended to that of the first transmit antenna. Thismay help in estimating channel responses with large delay spread whichother wise may not be estimated.

The various embodiments of the invention provide a method and system forchannel estimation in a block transmission system in which the channelresponses are estimated based on at least one weighted averaged delayprofile. The weighted averaged delay profile is calculated by performingat least one of a spatial weighted average and a time domain weightedaverage. As a result, the quality of channel estimates of lesser trainedtransmit antennas improves. Also if all the transmit antennas areequally trained, the quality of the channel estimates of each of themimproves as a result.

1. A method for performing channel estimation for each transmitantenna-receive antenna pair in a block transmission system, the methodcomprising the steps of: a. estimating at least one first delay profilefor each receive antenna corresponding to at least one first transmitantenna of a transmitter, the pilot density of the at least one firsttransmit antenna being significantly higher among a plurality oftransmit antennas of the transmitter; b. estimating at least one desireddelay profile for each receive antenna corresponding to each remainingtransmit antenna of the transmitter based on the at least first delayprofile, each remaining transmit antenna having pilot density lesserthan the at least one first transmit antenna; and c. estimating thechannel response of each receive antenna corresponding to each remainingtransmit antenna based on the corresponding desired delay profile;whereby, the delay spread of the channels corresponding to the remainingtransmit antennas that can be estimated is extended to that of the atleast one first transmit antenna.
 2. The method of claim 1, wherein theat least one first delay profile comprises a first tap delay profile. 3.The method of claim 1, wherein the at least one first delay profilecomprises a first power delay profile.
 4. The method of claim 1, whereinthe at least one first delay profile comprises a first channel impulseresponse (CIR) auto-correlation matrix.
 5. The method of claim 1,wherein the at least one desired delay profile comprises a desired tapdelay profile.
 6. The method of claim 1, wherein the at least onedesired delay profile comprises a desired power delay profile.
 7. Themethod of claim 1, wherein the at least one desired delay profilecomprises a desired channel impulse response (CIR) auto-correlationmatrix.
 8. A system for performing channel estimation for each transmitantenna-receive antenna pair in a block transmission system, the systemcomprising: a. a first delay profile estimator, the first delay profileestimator estimating at least one first delay profile for each receiveantenna corresponding to at least one first transmit antenna of atransmitter, the pilot density of the at least one first transmitantenna being highest among a plurality of transmit antennas of thetransmitter; b. a desired delay profile estimator, the desired delayprofile estimator estimating at least one desired delay profile for eachreceive antenna corresponding to each remaining transmit antenna of thetransmitter based on the at least first delay profile, each remainingtransmit antenna having pilot density lesser than the at least one firsttransmit antenna; and c. a channel response estimator, the channelresponse estimator estimating the channel response of each receiveantenna corresponding to each remaining transmit antenna based on thecorresponding at least one desired delay profile; whereby, the delayspread of the channels corresponding to the remaining transmit antennasthat can be estimated is extended to that of the at least one firsttransmit antenna.
 9. The system of claim 8, wherein the first delayprofile estimator, the desired delay profile estimator and the channelresponse estimator can be integrated into a single module.